1. Field of the Invention
The present invention relates to a vehicle drive unit that quickly supplies a hydraulic pressure to a hydraulic servo upon restarting of an engine to enable, for example, rapid engagement of a friction engagement element that enables starting of a vehicle.
2. Description of the Related Art
In known art, a vehicle having a function to automatically stop an engine when predetermined conditions are satisfied during running of the vehicle (an idling stop function) has been put to practical use in order to achieve fuel saving, reduction in exhaust gas emission, noise reduction, and the like. In such a vehicle, the engine is stopped when all of conditions such as zero vehicle speed, accelerator OFF, and brake ON are satisfied.
When the engine is stopped, an oil pump generally connected to the engine is also stopped. Therefore, for example, oil that is supplied to a forward clutch (hydraulic servo) to be engaged during forward running is discharged through an oil passage, and the forward clutch is disengaged.
When predetermined restart conditions are satisfied such as when a driver depresses an accelerator pedal, the stopped engine is restarted and the oil pump is also restarted. At this time, the forward clutch needs to be engaged as soon as the engine is restarted. Engagement shock will occur if the forward clutch is engaged at a high engine speed.
Various technologies for preventing such engagement shock have been proposed. For example, in one of the technologies, an accumulator capable of accumulating a hydraulic pressure is provided in a branched manner in an oil passage that connects a forward clutch of an automatic transmission and an oil pump for generating a hydraulic pressure to supply the hydraulic pressure to the forward clutch (Japanese Patent Application Publication No. JP-A-2000-313252). When the engine is restarted, the hydraulic pressure accumulated in the accumulator is supplied to the forward clutch. Thus, generation of engagement shock is prevented, and engine restart capability is improved.
In another technology, an accumulator is provided in a branched manner in a hydraulic circuit of an automatic transmission, and a check valve is provided on the oil pump side of the accumulator (Japanese Patent Application Publication No. JP-A-H8-14076, and Japanese Patent Application Publication No. JP-A-2005-226802). With this structure, a hydraulic pressure is constantly supplied from the accumulator to a forward clutch while the engine is stopped. The forward clutch in the automatic transmission is thus constantly maintained in an engaged state.
In the technology described in Japanese Patent Application Publication No. JP-A-2000-313252, however, the hydraulic pressure accumulated in the accumulator cannot be efficiently supplied to the forward clutch (hydraulic servo) in a short time when the engine is restarted. The reason why this problem occurs is as follows: when the hydraulic pressure accumulated in the accumulator is supplied to the forward clutch (hydraulic servo), the hydraulic pressure is supplied not only to the forward clutch (hydraulic servo) but also to a primary regulator valve and leaks through the primary regulator valve.
Moreover, a second accumulator, a second switch valve, and a branched oil passage are provided before the forward clutch (hydraulic servo). These elements increase a fluid resistance and an oil passage length when the hydraulic pressure accumulated in the accumulator is supplied to the forward clutch (hydraulic servo). This increases the time it takes until the hydraulic pressure is supplied from the accumulator to the forward clutch (hydraulic servo). This is one of the reasons why the hydraulic pressure accumulated in the accumulator cannot be efficiently supplied to the forward clutch (hydraulic servo) in a short time.
In the technology described in Japanese Patent Application Publication No. JP-A-H8-14076, and Japanese Patent Application Publication No. JP-A-2005-226802, on the other hand, the hydraulic pressure is constantly supplied from the accumulator to the forward clutch while the engine is stopped, whereby the forward clutch in the automatic transmission is constantly maintained in an engaged state. In this case, engagement shock can be eliminated because hydraulic pressure supply is not delayed. In this technology, however, the accumulator needs to have an increased capacity. Moreover, an outlet (turbine) side of a torque converter is stopped when the engine is started. Therefore, rotation of the engine needs to be absorbed by oil of the torque converter. The engine is thus restarted while stirring the oil of the torque converter. This imposes a large burden on the starter system, causing additional problems such as need for a larger starter.